1. Field of the Invention
The present invention relates to the field of integrated image sensors.
2. Description of the Related Art
An integrated image sensor is used to convert light impinging on the sensor into electrical signals. An image sensor typically includes one or more (e.g., an array of) photoelements such as photodiodes, phototransistors, or other types of photodetectors, where electrical signals are generated via the well-known photoelectric effect. These signals may then be used, for example, to provide information about light intensity, color, or the optical image focused on the sensor. One common type of image sensor is a CMOS image sensor.
FIG. 1 shows a schematic top view of a conventional CMOS image sensor 100 implemented in a single integrated circuit or chip. Sensor 100 comprises a photoelement array 102, a decoding(buffer area 104, and control, processing, and input(output (I/O) circuitry 106. Photoelement array 102 comprises an array of photoelements and associated circuitry such as switches and amplifiers. Each photoelement and its associated circuitry are collectively referred to as a pixel.
FIG. 2A shows a schematic top view of part of integrated CMOS image sensor 100 of FIG. 1. In particular, FIG. 2A depicts a representative 2xc3x972 region of individual pixels 202 of photoelement array 102 of FIG. 1. Each pixel 202 comprises a photoelement 204, its associated circuitry 206, an optional microlens 208, and an optional color filter 209. Individual pixels are separated by pixel boundaries 210.
FIG. 2B shows a schematic cross-sectional view of part of a single pixel 202 of FIG. 2A comprising a silicon substrate 212, a silicon oxide layer 214, and contact and interconnect metal structures 216. Oxide layer 214 is deposited onto substrate 212 and is typically a few microns thick, with metal structures 216 formed within layer 214. Representative structures (e.g., various p and n doped regions) for photoelement 204 and for a MOSFET transistor 218 of associated circuitry 206 of pixel 202 are shown in FIG. 2B as well. Filter 209 is attached to layer 214. Microlens 208 is placed over filter 209 and positioned to have its focal point inside photoelement 204.
The fraction of the layout area of each pixel that detects light is called the optical fill factor. The fill factor is less than 100% because some of the pixel area is used by other circuitry, such as associated circuitry 206 of FIG. 2A. Microlens 208 concentrates the incoming light onto photoelement 204 thereby improving the fill factor and consequently the sensitivity of image sensor 100. The area above photoelement 204 is substantially free of interconnect metal structures 216 to improve the quantum efficiency of the pixel (defined as the ratio of the number of collected photoelectrons (or photo-holes) to the number of incident photons). Greater quantum efficiency also improves the sensitivity of image sensor 100.
Image sensors such as image sensor 100 of FIG. 1 are prone to image degradation due to several sources of noise and/or spurious signals. One problem is charge leakage from photoelement 204, schematically represented in FIG. 2B by arrow 250. Due to the doping profiles of the edges of the photoelement, its periphery has a disproportionately large capacitance and electrical field. Charge stored in this area of the photoelement is susceptible to leakage into the bulk of the silicon substrate. One other problem associated with the periphery of the photoelement is capture by the photoelement of spurious photocurrent generated by light incident on associated circuitry 206, schematically represented in FIG. 2B by arrow 260. Charge leakage out of or into the photoelement can introduce errors into the electrical signal generated by the pixel and degrade the quality of the image captured by the image sensor.
One additional problem inherent to the image sensor stricture of FIG. 2B is blooming. Blooming is an overflow of charge from an oversaturated pixel to an adjacent pixel in the pixel array. Because each photoelement has a limit as to how much charge it can store, extra photogenerated charge may flow from the photoelement into the substrate, migrate to the pixel boundary, and transfer to an adjacent pixel where it may eventually be captured by the unsaturated photoelement in that pixel. This process is schematically represented in FIG. 2B by arrow 270. In particular, blooming is a problem for high contrast images (e.g., a very bright edge against a virtually black background) and is typically visible as either a vertical streak or white halo extending for several pixels.
One more problem with the image sensor structure of FIG. 2B is optical crosstalk. One way for the optical crosstalk to be introduced is when light enters a pixel through a color filter of an adjacent pixel (such as filter 209xe2x80x2 of FIG. 2B) and strikes the photoelement (such as photoelement 204 of FIG. 2B). This can result in the loss of color purity in an image. A different way for the optical crosstalk to occur is when light incident at one pixel is deflected or scattered and eventually captured by another pixel. Multiple reflections off of interconnect metal structures (such as metal structures 216 of FIG. 2B), various interfaces, and microlenses and waveguide properties of the oxide layer are largely responsible for this type of the optical crosstalk. Sample optical paths contributing to the optical crosstalk are schematically shown by certain thin arrows in FIG. 2B.
Optical and electrical noise and spurious signals degrade image quality and create artifacts in the image sensor""s output.
Embodiments of the present invention are directed to techniques for reducing noise and spurious signals in integrated image sensors by which at least some of the optical and/or electrical pathways responsible for generating the same are either inhibited or eliminated. Reduction of optical and/or electrical noise and of spurious signals improves image quality and helps to eliminate artifacts in the image sensor""s output. It also boosts the image sensor""s performance in low-light imaging applications where improved signal-to-noise ratio allows for longer exposure times.
According to one embodiment, the present invention is an integrated circuit having an image sensor, wherein the image sensor has an array of one or more pixels, wherein at least one pixel in the array comprises (a) a photoelement formed on a substrate and configured to generate an electrical signal in response to incident light; and (b) associated circuitry formed on the substrate and configured to process the electrical signal generated in the photoelement. At least part of the photoelement and at least part of the associated circuitry are formed within a common insulating layer formed on the substrate, wherein a portion of the common insulating layer corresponding to the photoelement has a thickness different from a thickness of a portion of the common insulating layer corresponding to the associated circuitry.
According to another embodiment, the present invention is an integrated circuit having a digital image sensor, wherein the digital image sensor has an array of one or more digital pixels, wherein at least one digital pixel in the array comprises (a) a photoelement formed on a substrate and configured to generate a digital electrical signal in response to incident light; (b) associated circuitry formed on the substrate and configured to process the digital electrical signal generated in the photoelement; and (c) one or more insulating structures formed on the substrate and configured to inhibit flow of electricity between at least one of (1) the photoelement and the associated circuitry and (2) the pixel and an adjacent pixel in the array.